The present invention relates to a signal switching circuit, and more particularly to a signal switching circuit provided with an amplifying function.
In a prior art signal switching circuit, a plurality of amplifiers are provided, corresponding in number to the number of input signals. Outputs of these amplifiers are commonly connected. A particular input signal is amplified by the associated amplifiers thereby providing an amplified output. In actual operation, all amplifiers except one are disabled.
FIG. 1 is a circuit diagram showing such a signal switching circuit in which amplifiers 1 and 2 corresponding respectively to input signals on terminals IN-1 and IN-2 are provided with the outputs of the two amplifiers coupled to a common output terminal OUT. In response to control signals on terminals CONT-1 and CONT-2 applied respectively to the amplifiers 1 and 2, one of the two input signals is selectively amplified by the associated amplifier and the resultant output is obtained at the common output terminal OUT. Since the circuit arrangement of the amplifier 2 is similar to that of the amplifer 1, only the circuit arrangement of the amplifier 1 is shown in FIG. 1.
In the amplifier 1, the input signal on the terminal IN-1 is applied as a differential input to the first differential amplifier including a pair of differential transistors Q.sub.1 and Q.sub.2. Voltages developed across collector resistors R.sub.1 and R.sub.2 of the transistors Q.sub.1 and Q.sub.2 are applied as differential inputs to the second differential amplifier. A current source for the first differential amplifier is provided which includes a transistor Q.sub.3, a resistor R.sub.3, and a bias supplying source E. The base potential of the transistor Q.sub.3 is controlled by a switching transistor Q.sub.4 to thereby cause the differential amplifier to be either enabled or disabled. To this effect, a control signal is applied to the base of the transistor Q.sub.4. Resistor R.sub.4 is provided for restricting the current applied to the base of the transistor Q.sub.3.
The second differential amplifier including a pair of transistors Q.sub.5 and Q.sub.6, emitter resistors R.sub.5 and R.sub.6, and a current mirror circuit, operates as an active load for the transistors Q.sub.5 and Q.sub.6. The current mirror circuit includes a transistor Q.sub.7, a diode D.sub.1, and resistors R.sub.7 and R.sub.8. Between the collectors of the transistors Q.sub.6 and Q.sub.7, series-connected diodes D.sub.2 and D.sub.3 are arranged. The voltage developed across the series-connected diodes D.sub.2 and D.sub.3 provides base driving signals for output push-pull transistors Q.sub.8 and Q.sub.9. The output transistors Q.sub.8 and Q.sub.9 are connected as complementary transistors with emitters being commonly connected to each other in an emitter follower configuration. The common emitter junction point is coupled to the output terminal OUT to which the output of the other amplifier is also coupled.
In the circuit arrangement shown in FIG. 1, when the control signal on the terminal CONT-1 is at a high level and the control signal on the terminal CONT-2 is a low level, the amplifier 1 is disabled. Therefore, only the input signal on the terminal IN-2 is amplified by the amplifier 2 and the output thereof provided at the output terminal OUT. In this operation, the circuit of the amplifier 1 forms a part of a load for the amplifier 2.
Here, the impedance of the amplifier 1 from the output terminal OUT is extremely large in the low frequency range. Thus, the amplifier 1 has substantially no effect upon the amplifying operation of the amplifier 2. In the high frequency range, however, stray capacitances existing between the base of the transistor Q.sub.8 and ground and between the base of the transistor Q.sub.9 and ground are not negligible. The impedance Z of the amplifier 2 is equivalent to that obtained from the circuit shown in FIG. 2. In the circuit of FIG. 2, D.sub.8 and D.sub.9 are base-emitter PN junction diodes of the emitter follower transistors Q.sub.8 and Q.sub.9, respectively. Specifically, in the high frequency range, the impedance Z of the amplifier 2 causes distorion of the signal on the output terminal OUT due to the non-linearity of the diodes D.sub.8 and D.sub.9.